CONICET | Buscador de Institutos y Recursos Humanos

The Quaternary Payún Matrú volcano is a long-lived edifice that developed a summit caldera 8 km wide, withabundant pre- and post-caldera volcanic activity. It is the main volcano of the Payún Matrú Volcanic Field,which is located in the back-arc Payenia Basaltic Province, at mid-western Argentina. The composition ofPayún Matrú is mainly trachytic, with lesser amounts of trachyandesitic and basaltic trachyandesitic lavas. ThePayún Matrú Volcanic Field includes also the Payún Liso stratovolcano and more than 200 monogenetic basalticcones and associated lava flows that are located east and west of the caldera. The aimof this work is to show thePayún Matrú evolution based on the mineralogical and textural characterization and to make inferences on thetrigger mechanism for the explosive eruption which leads to the caldera formation. Some intermediate lavasand trachytes include phenocrysts with contrasting textures and composition, such as inverse zoning in plagioclaseand clinopyroxenes, sieved plagioclaseswith amore calcic rim and calcic plagioclaseswith amore sodic rim,indicating that they could not have formed together and suggesting magma mixing processes. In addition, a fewlavas show mafic enclaves or texturally different groundmasses with fluidal contacts, indicating mingling betweentwo magmas. These lavas are found in the pre-caldera stage and the early post-caldera stage of PayúnMatrú. The trachytic pyroclastic deposits related to the caldera collapse do not exhibit evidence of hybridization,although they present a minor proportion of dissolved feldspars and biotite replaced by anhydrous minerals, suggestingthat a restricted zone of the chamber was reheated. This may be explained by a mafic recharge eventwhere part of the host trachytic magma was heated, but without a complete homogenization between thehost and recharging magmas. These features, in addition to the abundant basaltic volcanism east and west ofthe caldera, suggest that the trigger mechanism for the explosive eruption was the injection of basaltic magmainto the trachytic chamber of Payún Matrú.